Virus replication in eukaryotic cells involves the utilization of many host cell functions by a foreign genome. However, in most cases genetic analysis of the host functions is not feasible at present. Therefore, the killer RNA plasmid and 2-micron DNA plasmid of Saccharomyces cerevisiae are being studied as experimental models. The killer plasmid is one of three double-stranded RNA molecules found in spherical intracellular virions (killer virus). Replication of the killer virus is known to require at least 26 host genes, and expression of the plasmid-induced toxin and toxin-resistance requries at least three additional host genes. Purified double-stranded RNA-containing virions have been found to contain two distinct DNA-independent RNA polymerase activities. Biochemical and genetic studies will test the hypothesis that one of these enzymes is a plasmid replicase while the other is a transcriptase. Host and plasmid mutants in the killer system will be studied biochemically and new mutants will be obtained by direct screening for enzymes acting on this plasmid. Viable yeast spheroblasts and other permeabilized organisms will be tested as recipients of plasmids or plasmid-bearing particle components and the prooducts of RNA replication enzymes. Similar studies are being initiated on the 2-micron DNA plasmid, a covalently closed circular double-stranded genome of unknown function which appears to be associated with intracellular particles in most yeast strains. The replication of this plasmid is know to require some genes also required for chromosomal DNA synthesis. The enzymology of 2-micron DNA replication will be studied with the goal of biochemically characterizing the replication of a defined DNA molecule in a eukaryote. Genetic studies will follow such characterization, using mutants now available blocked in host cell division and plasmid replication as well as new mutants to be sought by screening for defects in enzymes acting on 2-micron DNA.